Etymologies

Middle English redoubtabel, from Old French redoutable, from redouter, to dread : re-, re- + douter, to doubt, fear; see doubt.

(American Heritage® Dictionary of the English Language, Fourth Edition)

French redoutable (spelled redoubtable in early modern French). (Wiktionary)

Examples

General Byng's attack covered the whole length of what had become known as the redoubtable and supposedly impregnable "Hindenburg line," so called because it had been established by that greatest of all German military geniuses, Field Marshal von Hindenburg.

Lucien watched this Dauriat, who addressed Finot with the familiar tu, which even Finot did not permit himself to use in reply; who called the redoubtable Blondet “my boy,” and extended a hand royally to Nathan with a friendly nod.

But, Charles having got over to Scotland where the men of the Solemn League and Covenant led him a prodigiously dull life and made him very weary with long sermons and grim Sundays, the Parliament called the redoubtable Oliver home to knock the Scottish men on the head for setting up that Prince.

League and Covenant led him a prodigiously dull life and made him very weary with long sermons and grim Sundays, the Parliament called the redoubtable Oliver home to knock the Scottish men on the head for setting up that Prince.

Lucien watched this Dauriat, who addressed Finot with the familiar tu, which even Finot did not permit himself to use in reply; who called the redoubtable Blondet "my boy," and extended a hand royally to